A patch clamp method is known as a conventional method for clarifying the function of ion channels present in a cell membrane, using the electrical activity of the cell as an index, or screening (testing) a medicine.
However, the patch clamp method requires special technologies and techniques in preparation and operation of a micropipette for sucking a micropart (referred to as a patch) of a cell membrane. Therefore, the method takes much time to measure one specimen, and thus is not suitable for applications of screening a large amount of candidate chemical compounds at high speed.
On the other hand, in recent years, a flat panel-shaped microelectrode probe made by a micromachining technology has been developed. A method using such a flat panel-shaped microelectrode probe is suitable for the automation system that does not require insertion of micropipettes into individual cells.
As this type of technology, Patent Literature 1, for example, discloses the following technology: a plurality of through-holes are formed through a cell-holding membrane, specimen cells are attached to the openings of the through-holes, and the voltage-dependent ion channel activity of the specimen cells is measured with a measuring electrode disposed below the through-holes.
Non Patent Literature 1 discloses the following technology: a hole of 2.5 μm is formed through silicon oxide cell-holding membrane, an HEK293 cell, a type of cultured human cell line, is held in this hole with high adherence ensured therebetween, and the extracellular potential is measured with high accuracy.
Further, Patent Literature 2 discloses a cell electrophysiological sensor where a cell is trapped (captured) in a recess formed in a cell-holding membrane, and the potential difference between a reference electrode and a measuring electrode disposed on both sides of the cell-holding membrane is obtained to analyze the functionality of the ion channels in the cell, for example.
In this manner, in order to screen a large amount of candidate chemical compounds at high speed, the potential difference between the reference electrode and the measuring electrode is obtained with a cell trapped in a through-hole or the recess.
As such a conventional cell electrophysiological sensor, the electrophysiological sensor disclosed in Patent Literature 2 is detailed with reference to FIG. 7. As shown in FIG. 7, conventional cell electrophysiological sensor 31 has cell-holding membrane 32, recess 33 formed in the top surface of cell-holding membrane 32, and through-hole 34 connecting the bottom portion of recess 33 and the bottom surface of cell-holding membrane 32. Further, the sensor has reference electrode 35 disposed above cell-holding membrane 32 and measuring electrode 36 disposed inside of through-hole 34. Measuring electrode 36 is coupled to a signal detector via wiring 37. Cell-holding membrane 32 is disposed inside of well 38.
Next, a measuring method using cell electrophysiological sensor 31 is described. First, a cell and electrolytic solution 40 are placed in well 38. The cell is trapped and held by recess 33. The cell held by recess 33 is referred to as specimen cell 39 hereinafter.
During measurement, specimen cell 39 is sucked with a suction pump, for example, from the downward direction of through-hole 34, and held onto the opening of through-hole 34 in intimate contact therewith. That is, through-hole 34 has a function similar to that of the tip hole of a glass pipette. The functionality and pharmacodynamic reaction of the ion channels in specimen cell 39 are analyzed by measuring the voltage or current between reference electrode 35 and measuring electrode 36 before and after the reaction and obtaining the potential difference between the inside and outside of the cell.
However, conventional cell electrophysiological sensor 31 has errors in the measurements of the potential difference between reference electrode 35 and measuring electrode 36, so that the measuring reliability of cell electrophysiological sensor 31 is degraded.
This is caused by the following reason. Bubbles are likely to remain in a portion of the inner wall surface of well 38 having low hydrophilic properties, or a portion having asperities on the periphery of through-hole 34. The resistance of these bubbles is so large that the presence of the bubbles causes variations in measurements.
In particular, bubbles occurring in the vicinity of through-hole 34 are the factors for causing great variations in the measurements of the current or voltage detected in measuring electrode 36.
[Patent Literature 1] Japanese Translation of PCT Publication No. 2002-518678
[Patent Literature 2] International Publication No. 02/055653 Pamphlet
[Non Patent Literature 1] T. Sordel et al, Micro Total Analysis Systems 2004, p 521-522 (2004)